FIG. 1 is an illustration of a prior art system for charging conventional battery cells (depicted as CELL 1 and CELL 2). In this prior art system, two cells (CELL 1 and CELL 2) are charged by way of a source current (Ico) supplied by a conventional charging system (not shown). Prior art systems generally select the source current Ico according to the cutoff current of one of the cells. The reader's attention is directed to FIG. 2, which provides a diagram depicting the relationship of cycle-life (i.e., the number of functional charge and discharge cycles of a conventional battery cell) and the charging capacity of said cell as a function of source voltage and cutoff current. From this illustration, the cutoff current of a cell is typically 40 mA to 50 mA and 4.2 volts.
Prior art systems such as shown in FIG. 1 set the source current Ico to cutoff current shown in FIG. 2. From the illustration of FIG. 1, CELL 1 and CELL2 have asymmetric capacities of 500 mAh (milli-Ampere hours) and 1000 mAH, respectively. The cutoff current at each cell can be determined from a product of the source current Ico and the ratio of the capacity of the cell in question to the total capacity of the cells. Accordingly, the cutoff current of CELL 1 (Ic1) is 13.3 mA, while the cutoff current of CELL 2 (Ic2) is 26.7 mA.
Referring back to FIG. 2, four curves are shown (10, 12A-B, and 14) at a variety of source voltages and cutoff currents. Starting with curve 10, a source voltage of 4.3V at a cutoff current of 40 mA provides a higher capacity charge (950 mAh), but a shorter cycle-life (500 cycles) than curves 12 and 14. Curve 12A provides a charge capacity of 875 mAh and a cycle-life of 750 cycles at a lower source voltage (4.2V), but the same cutoff current (40 mA). Thus, the lower source voltage (4.2V) provides a longer cycle-life, but a lower charge capacity. Curve 14 provides a charge capacity of 790 mAh and a cycle-life of greater than 1000 cycles at a source voltage of 4.1V and cutoff current of 40 mA.
From these curves 10-14 it should be apparent that varying the source voltage results in an inverse relationship between charge capacity and cycle-life. It is also important to note that when the cutoff current is significantly reduced, the cycle-life of the battery cell is significantly impacted. Curve 12B shows that when the cutoff current is reduced by half (20 mA) the cell's cycle-life is impacted by 20% (i.e., a cycle-life of 600 cycles—a reduction of 150 cycles from curve 12A). This latter effect has an undesirable impact on the cycle-life of parallel cells of the prior art system of FIG. 1.